1. Field of the Invention
The present invention relates to a technique of adequately changing over the setting of compression ratio in an internal combustion engine with a variable compression ratio.
2. Description of the Related Art
The internal combustion engine is small in size but is capable of outputting relatively large power. Because of these advantages, the internal combustion engine is widely used as the power source of various transportations including automobiles, boats and ships, and aircraft, and as the power source of diverse stationary equipment and machinery. The principle of the internal combustion engine makes the compressed air-fuel mixture subjected to combustion in a combustion chamber and converts the pressure of combustion into mechanical power.
In the internal combustion engine, the higher compression ratio, which represents the ratio of compression of the air-fuel mixture, theoretically enhances the thermal efficiency. In the actual state, however, the higher compression ratio undesirably heightens the potential for the occurrence of abnormal combustion called knocking. Driving the internal combustion engine in the state of knocking may damage the internal combustion engine. In the internal combustion engine with setting of a high compression ratio, especially in a driving area of high load having the high potential for the occurrence of knocking, the ignition timing is delayed from its optimum timing to prevent the occurrence of knocking. The delayed ignition timing effectively prevents the occurrence of knocking but naturally lowers the output of the internal combustion engine.
Various techniques of changing the compression ratio set in the internal combustion engine have been proposed by taking into account these findings. Some of these techniques are disclosed in, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 62-258153 and No. 63-159642. The technique described in these cited references changes over the setting of the compression ratio according to the driving conditions of the internal combustion engine. The control procedure sets a high compression ratio in the driving conditions of low and medium load having the low potential for the occurrence of knocking, while setting a low compression ratio in the driving conditions of high load having the high potential for the occurrence of knocking. The internal combustion engine is driven at the high compression ratio in the driving area of low and medium load. This attains the high thermal efficiency. The internal combustion engine is driven at the low compression ratio in the driving area of high load. This assures a large output of the internal combustion engine while preventing the occurrence of knocking.
The changeover of the compression ratio set in the internal combustion engine enables the simultaneous improvement of the thermal efficiency and the maximum output, but has the drawbacks discussed below. The changeover of the setting of the compression ratio in the internal combustion engine requires some energy. Frequent changeover of the compression ratio undesirably consumes large energy and may lower the total thermal efficiency of the internal combustion engine. The changeover of the setting of the compression ratio also takes some time. Frequent changeover of the compression ratio may give the sense of discomfort to the operator of the internal combustion engine. The simple changeover of the compression ratio varies the output of the internal combustion engine and may give the sense of discomfort to the operator of the internal combustion engine. The complicated control strategy is thus required to prevent the variation in output of the internal combustion engine. A technique of eliminating such drawbacks has highly been demanded.
The object of the invention is thus to eliminate the drawbacks of the prior art techniques without damaging the advantages of an internal combustion engine with a variable compression ratio.
The invention is directed to an internal combustion engine and a control method of the internal combustion engine, in order to attain at least part of the above and the other related objects. The control technique of the invention detects a torque demand required to an output shaft of the internal combustion engine and compares the detected torque demand with the threshold torque a threshold torque, which is set in advance for changeover of setting of the compression ratio in the internal combustion engine. The control technique distributes the torque demand into a first torque value to be transmitted between the motor and the output shaft and a second torque value to be output from the internal combustion engine, based on a result of the comparison. The control technique also specifies a desired setting of the compression ratio, based on the result of the comparison. The control technique then changes over the setting of the compression ratio in the internal combustion engine according to the specification of the desired setting of the compression ratio, while controlling the motor to regulate torque transmission between the motor and the output shaft according to the first torque value. This arrangement sets the appropriate compression ratio in the internal combustion engine and drives the internal combustion engine to output the required torque.
One preferable application distributes the torque demand in such a manner as to reduce a frequent change of the torque division to be output from the internal combustion engine across the preset threshold torque, in response to a variation in detected torque demand. The setting of the compression ratio in the internal combustion engine is not changed over, when the torque demand only slightly exceeds the threshold torque. Such control effectively prevents frequent changeover of the setting of the compression ratio in the internal combustion engine. This arrangement desirably saves energy required for the frequent changeover of the compression ratio and thus enhances the total efficiency of the internal combustion engine. This arrangement also keeps the operator of the internal combustion engine free from the sense of discomfort.
The changeover of the compression ratio takes some time. It is accordingly difficult to quickly increase the output torque of the internal combustion engine by changing over the compression ratio, in response to an increase in torque demand. The control of increasing the output torque of the motor without changing over the compression ratio, on the other hand, advantageously responds to the increase in torque demand.
In the internal combustion engine and the corresponding control method, one preferable embodiment stores a control area to specify setting of the compression ratio in the internal combustion engine and state of torque output from the motor to the output shaft, corresponding to at least the torque demand required to the output shaft, and gives an instruction to change over the compression ratio and control the motor according to the specification of the control area. It is preferable that multiple control areas are stored, which include at least a first control area to specify setting of a predetermined compression ratio in the internal combustion engine and selection of no torque output as the first torque value, corresponding to a range of lower torque demand than the threshold torque, and a second control area to specify setting of the predetermined compression ratio in the internal combustion engine and selection of the first torque value, corresponding to a range of higher torque demand than the threshold torque.
When the torque demand exceeds the threshold torque, the motor is controlled to output the torque according to the specification of the control area. Such control restricts the output torque of the internal combustion engine to be not greater than the threshold torque. This arrangement effectively prevents frequent changeover of the compression ratio.
The above embodiment may further store a third control area to specify setting of a lower compression ratio than the predetermined compression ratio in the internal combustion engine, corresponding to a range of higher torque demand than the torque demand range of the second control area. The control may depend upon accumulation of electric power, which is supplied to the motor. When the observed amount of power accumulation is not greater than a preset threshold value, the control procedure gives an instruction to change over the setting of the compression ratio in the internal combustion engine and control the motor according to the specification of the third control area, in response to the detected torque demand even belonging to the second control area.
In the insufficient state of charge, the compression ratio set in the internal combustion engine is lowered to increase the output torque of the internal combustion engine. This desirably reduces the output torque of the motor and the power consumption.
In the internal combustion engine and the corresponding control method, another preferable embodiment counts elapse of a preset time after the detected torque demand exceeds the threshold torque. The control procedure controls the motor to output an increased torque from the motor to the output shaft until the torque demand exceeding the threshold torque continues for the preset time. When the torque demand exceeding the threshold torque continues for the preset time, the control procedure changes over the setting of the compression ratio in the internal combustion engine to increase the output torque of the internal combustion engine, while controlling the motor to decrease the output torque from the motor to the output shaft.
When the torque demand exceeds the threshold torque only for a shorter time than the preset time, the control procedure does not change over the setting of the compression ratio in the internal combustion engine but increases the output torque of the motor. This arrangement effectively prevent frequent changeover of the setting of the compression ratio. When the torque demand exceeding the threshold torque continues for or over the preset time, on the other hand, the control procedure changes over the setting of the compression ratio in the internal combustion engine to increase the output torque of the internal combustion engine, while decreasing the output torque of the motor. This desirably reduces the power consumption.
When the observed amount of power accumulation is not greater than a preset threshold value, the control procedure may change over the setting of the compression ratio in the internal combustion engine to increase the output torque of the internal combustion engine, while decreasing the output torque of the motor before elapse of the preset time, for example, after elapse of a shorter time than the preset time or immediately after the torque demand exceeds the threshold torque. The preset time may be regulated according to the state of charge.
This arrangement effectively reduces the power consumption in the insufficient state of charge.
At least part of the object discussed above and the other related objects is also attained by another internal combustion engine and a control method of the internal combustion engine. The control technique regulates torque transmission between the motor and the output shaft in the course of changeover of the compression ratio set in the internal combustion engine, and thereby reduces a variation in output torque of the internal combustion engine to be not greater than a predetermined level. This arrangement implements changeover of the compression ratio without giving the sense of discomfort to the operator of the internal combustion engine.
In this internal combustion engine of the invention and the corresponding control method, one embodiment controls a driving state of the internal combustion engine with setting of a control amount unchanged in the course of the changeover of the compression ratio. Here the setting of the control amount is fixed to a specific control amount corresponding to the higher compression ratio between a previous compression ratio before the changeover and a new compression ratio after the changeover.
The higher compression ratio generally heightens the potential for the occurrence of abnormal combustion called knocking in the internal combustion engine. The compression ratio is changed over, while the setting of the control amount of the internal combustion engine is fixed to the specific control amount corresponding to the higher compression ratio between those before and after the changeover. This effectively prevents the occurrence of knocking in the course of the changeover.
In this internal combustion engine of the invention and the corresponding control method, the control procedure may change over the compression ratio, while the setting of the control amount of the internal combustion engine is fixed to a specific control amount corresponding to a high compression ratio in the course of changeover from the high compression ratio to a low compression ratio.
This arrangement only requires storage of the previous setting of the control amount during the changeover of the compression ratio, thus advantageously simplifying the control procedure.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings.